1. Field of the Invention
The present invention relates to formulations of nanoparticulate human immunodeficiency virus (HIV) protease inhibitor drug substances comprising a cellulosic surface stabilizer. The nanoparticulate formulations have an increased rate of dissolution in vitro, an increased rate of absorption in vivo, a decreased fed/fasted ratio variability, and a decreased variability in absorption. The present invention is also directed to methods of making the novel formulations. In particular, nanoparticulate formulations of HIV type 1 (HIV-1) and type 2 (HIV-2) protease inhibitors employing cellulosic stabilizers are disclosed.
2. Description of the Related Art
It is known that with an increase in surface area of a particulate therapeutic agent, the rate of dissolution of the agent increases. Such an increase in surface area can be obtained by decreasing the particle size of the agent. Increasing the rate of dissolution of a therapeutic agent is often desirable because it can result in an increased rate of absorption in vivo, increased bioavailability, and decreased variability in absorption of the agent.
Bioavailability is the degree to which a drug becomes available to the target tissue after administration. Many factors can affect bioavailability, including the dosage form and dissolution rate of the drug. Poor bioavailability is a significant problem encountered in the development of pharmaceutical compositions, particularly those containing an active ingredient that is poorly soluble in water. Poorly water soluble drugs tend to be eliminated from the gastrointestinal tract before being absorbed into the circulation of the patient. Moreover, poorly water soluble drugs tend to be unsafe for intravenous administration techniques, which are used primarily in conjunction with fully soluble drug substances.
As noted above, it is known that by increasing the surface area of a particulate drug, such as by decreasing the particle size of the drug, the rate of dissolution of the particulate drug is increased. Consequently, efforts have been made to control the size and size range of drug particles in pharmaceutical compositions and methods of making fine particulate drugs have been studied. For example, dry milling techniques have been used to reduce particle size and thereby influence drug absorption. However, in conventional dry milling, as discussed by Lachman et al., "The Theory and Practice of Industrial Pharmacy," Milling, 45 (1986), the limit of fineness is reached at about 100 microns (100,000 nm) when material cakes on the milling chamber. Lachman et al. also note that wet grinding is useful to further reduce particle size, but that flocculation restricts the lower particle size limit to approximately 10 microns (10,000 nm). Commercial airjet milling techniques have provided particles ranging in average particle size from as low as about 1 to 50 .mu.m.
Other techniques for preparing pharmaceutical compositions to aid in dissolution and increase bioavailability include loading drugs into liposomes or polymers, such as during emulsion polymerization. However, such techniques exhibit problems and limitations. For example, a lipid soluble drug is often required in preparing suitable liposomes. Further, unacceptably large amounts of the liposomes or polymer are often required to prepare unit drug doses. Further still, techniques for preparing such pharmaceutical compositions tend to be complex.
One solution to the problem of preparing water-insoluble drugs previously suggested is to provide stable dispersible drug particles in the submicron size range, as described in U.S. Pat. No. 5,145,684 ("the '684 patent"), specifically incorporated herein by reference. The submicron sized particles can be prepared by wet milling in the presence of grinding media in conjunction with a surface stabilizer. Such particles can be readily prepared, do not appreciably flocculate or agglomerate due to interparticle attractive forces, and do not require the presence of a cross-linked matrix. In the '684 patent, the inventors hypothesized that the surface stabilizer hinders the flocculation and/or agglomeration of the drug particles by functioning as a mechanical or stearic barrier between the particles, thereby minimizing the close, interparticle approach necessary for agglomeration and flocculation. However, as noted in the '684 patent, not all stabilizers will function to produce a nanoparticulate composition of any drug.
Drug substances which have been difficult to formulate prior to the present invention because of their low solubility include HIV protease inhibitors, which are useful in the treatment of AIDS (acquired immune deficiency syndrome), caused by HIV infection.
AIDS was first recognized in 1981 as a clinical syndrome consisting of opportunistic infection and/or neoplasia associated with unexplained immunodeficiency. Shaw et al., AIDS: Etiology, Diagnosis, Treatment, and Prevention, 2nd Edition, DeVita, Jr. et al., eds., 11-31 (J.B. Lippincott Co., 1988). AIDS is a complex disease that includes progressive destruction of the immune system and degeneration of the central and peripheral nervous system. The discovery of HIV as the etiological agent of AIDS followed several years later. Id. at 12-13.
HIV-1 and HIV-2 are retroviruses, and like all retroviruses, they have a RNA-dependent DNA polymerase. In the life cycle of the HIV virus, the cell-free virion first binds to the target cell. Following virus adsorption, reverse transcription catalyzed by the viral RNA-dependent DNA polymerase generates a double-stranded DNA copy of the single-strand viral RNA genome. Subsequent expression of viral DNA is controlled by a combination of viral and host cellular proteins interacting with viral DNA regulatory elements.
One method of treating HIV infection targets the reverse transcriptase (RT). Drugs which interfere with the HIV RT are nucleoside analogs, which include AZT (azidothymidine), ddI (didanosine), and ddT (dithiothreitol). However, such drugs are only modestly effective and are plagued by severe side effects because they interfere with cellular enzymes similar to the HIV viral enzyme they target.
An alternative method of treating HIV infection targets the HIV protease. The HIV protease, also known as proteinase, is essential for the final assembly of viable viral particles in HIV replication. Basically, the HIV protease cuts and trims the proteins that give the virus its structure. More specifically, the HIV protease cleaves a number of proteins from the viral Gag-Pol polyprotein precursor, including the HIV protease, the reverse transcriptase and integrase encoded in the pol gene, and the matrix, capsid, and nucleocapsid encoded in the gag gene. If the Gag-Pol precursor molecule is not processed in this way, noninfectious particles are formed. More importantly, HIV protease is unlike other enzymes in human cells and, therefore, inhibition of the HIV protease should not interfere with normal human cellular action. Inhibition of the HIV protease therefore represents an attractive treatment of HIV infection.
However, the identification of HIV protease inhibitors with antiretroviral activity in vivo has been hampered by the poor bioavailability of many molecules in this class. For example, it has been reported that the following HIV protease inhibitors, which are some of the largest and most complex drugs in development or marketed, all suffer from low oral bioavailability: Hoffmann La-Roche's saquinavir (INVIRASE.RTM.), Vertex's VX-478, Merck's MK-639 (L-735,524) (also known as indinavir, and marketed under the trade name CRIXIVAN.RTM.), Agouron Pharmaceutical's AG1343, Abbott Labs' ABT-538, Upjohn Co.'s U-103,017, Dupont Merck's DMP-450, and National Cancer Institute's and Japan Energy's KNI-272. Early HIV Protease Inhibitors Difficult to Produce and Supplies Restricted, Biotechnology Information Institute Antiviral Agents Bulletin, March, 1995. It has also been reported that many HIV protease inhibitors, such as saquinavir, are poorly soluble and therefore not very good at getting into the bloodstream. Step By Step, The Economist Newspaper, Nov. 26, 1994, at 93. Moreover, early trials of an HIV protease inhibitor developed by Searle were stopped because the drug seemed unable to enter the body's cells. Id.
With insolubility and low bioavailability, HIV protease inhibitors have to be given in huge doses to effect results. Id. For example, it has been reported that saquinavir is administered at 1800 mg/day, and indinavir (MK-639) is administered at 2400 mg/day, while the average amount of drug used to treat most diseases is 10-30 mg/day. Id. Thus, the dosage of a typical HIV protease inhibitor can be up to 24 times that of a typical drug. The low oral bioavailability and rapid biliary excretion of many HIV protease inhibitors have limited their utility as potential therapeutic agents. Thaisrivongs et al., J. Med. Chem., 39:2400-10 (1996). Such large dose requirements also translate to expensive drug protocols for patients.
HIV protease inhibitors have also been found to have an extremely poor taste. The poor taste, in conjunction with the necessity of large doses of the drugs, has made it extremely difficult, if not impossible, to develop formulations of the drugs which are palatable to patients. Even large doses of sweeteners have been unsuccessful in masking the unpleasant taste of the drugs.
There is a need in the art for compositions of HIV protease inhibitors which provide for high bioavailability and which are soluble in water. In addition, there is a need in the art for methods of making such compositions. The present invention satisfies these needs.